This invention relates to the fabrication of p-n junction wafers in GaAs and GaAlAs compound semiconductors by the liquid phase epitaxy process. More particularly, it relates to a method of avoiding the occurrence of pyramidal protrusions on the final surface of such crystals when silicon is used as the significant impurity or dopant.
Liquid phase epitaxy growth is a well-known and widely practiced technique for the manufacture of p-n junction compound semiconductor devices, particularly for use as light-emitting diodes and other opto-electronic devices. In particular, silicon is known as a particularly useful dopant for GaAs and GaAlAs because of its amphoteric character. Silicon acts as a donor impurity when incorporated in the crystal during a higher temperature range and as an acceptor impurity when incorporated during a lower temperature range. The change from one condition to the other occurs at a temperature referred to as the "transition temperature" and is utilized in the liquid phase epitaxial process to fabricate an epitaxial layer having a p-n junction formed therein. This technique is well known.
In the standard liquid phase epitaxy process in which a plurality of substrate slices are treated simultaneously, the epitaxial layers are deposited by cooling the slices and melt in a temperature range from about 920.degree. Celsius to about 350.degree. Celsius, with the melt continuously in contact with the substrates, after which the substrates and contacting melt are moved to a cool portion of the reactor and effectively quenched.
However, when this practice is followed, the last-formed surface, which is of p-type conductivity, contains numbers of pyramidal protrusions which were found to consist largely of silicon, the significant impurity used in these materials. The occurrence of these pyramidal protrusions results in the formation of dark-line defects at dislocation networks surrounding the pyramids and thus degrades the optical characteristics of the devices. The removal of the pyramids by etching alleviates the bonding stresses associated with these pyramids, but there are residual dislocation networks which contribute to continuing optical degradation.
A reduction in the silicon content not only does not alleviate the occurrence of the silicon pyramids, but also affects the emission characteristics of the device. Accordingly, there is a need for a means for avoiding the occurrence of such pyramidal protrusions. It is important that such means not increase the fabrication costs of such devices by increasing the time required for processing excessively or by requiring the addition of costlier materials.